[Superman]

Rock in the tread?

No, I'm not kidding. Here is some elimination info: U-joint problems are much faster. That is, with a real u-joint and a drive shaft that runs to a rear differential, the drive shaft spins too fast. A balance problem would be a buzz, not a click.

Same with tranny noises. If this is purely speed-dependent, then it's a problem with a tire, wheel, brake rotor....that sort of thing.

Wheel bearing problems almost always make a rumbling sound. Well, not really rumbling. More of a growling sound. Growling/howling.

Rock in the tire tread. Kinda kidding. Kinda not. This might be something too simple for the inspectors to have noticed yet.